1. Field of the Invention
This invention relates to a PLL (phase-Locked Loop) circuit, and more particularly, to a PLL circuit adapted for use in frequency synthesizers communication equipment, or the like.
2. Description of the Related Art
FIG. 5 is a diagram showing the configuration of a conventional frequency synthesizer using a PLL circuit. In the figure, reference numeral 20 designates a PLLic (Phase-Locked Loop Integrated Circuit). The PLLic 20 has a prescaler, a programmable divider, a phase comparator, a charge pump, and the like, which are not shown, formed on a single chip. The phase comparator receives a reference frequency signal fr. Reference numeral 30 designates a loop filter (low-pass filter) for removing high-frequency components and noise components from the output signal of the PLLic 20. A voltage-controlled oscillator 40 (VCO) has an oscillation frequency that is controlled according to the output signal of the loop filter 30. An impedance matching circuit 50 provides impedance matching with a downstream circuit. The output signal of the impedance matching circuit 50 is applied to PLLic 20.
Referring to FIG. 6, voltage-controlled oscillator 40 includes a resonator circuit 42, a negative resistor circuit 44, and a buffer amplifier 46.
Downsizing of such a PLL circuit would be desirable. Since the shape of the integrated circuit cannot be modified, the loop filter 30 and the voltage-controlled oscillator 40 are expected to be downsized. However, there is a limitation on the simplification of the design of elements forming the circuit because the various conventioned characteristics of the circuit need to be maintained. There is also a limitation on the downsizing of individual elements forming the circuit. As a result, downsizing of the PLL circuit is extremely difficult.
Further, in the processing of high frequencies, the wiring on the circuit becomes a source of inductance components, which deteriorates the circuit characteristics. Therefore, consideration must be given to minimizing the inductance components between the voltage-controlled oscillator 40 and the PLLic 20. However, where the terminals of the PLLic 20 are predetermined, this in turn determines the positional relationship between the PLLic 20 and the voltage-controlled oscillator 40. This means that freedom in designing the circuit is limited, which gives a restriction in the reduction of the inductance components.
In general, characteristics of the voltage-controlled oscillator are largely determined by the resonator circuit and the negative resistor circuit. If these two circuits are included in the PLLic, the characteristics of the voltage-controlled oscillator become inferior due to the structure of the IC. Further in such a case, an excellent discrete components such as a piezoelectric resonator can not be used as a part of the resonator circuit.